The present invention generally relates to welding methods. More particularly, this invention is directed to a welding process that utilizes a hybrid laser arc welding technique in which laser beam welding and arc welding simultaneously occur in the same weld pool, and further utilizes a second laser beam welding technique that precedes the hybrid laser arc welding technique to increase the weld depth and/or promote the elimination of porosity and gas pockets in the resulting weld joint.
Low-heat input welding processes, and particularly high-energy beam welding processes such as laser beam and electron beam welding (LBW and EBW, respectively) operated over a narrow range of welding conditions, have been successfully used to produce crack-free weld joints in a wide variety of materials, including but not limited to alloys used in turbomachinery. An advantage of high-energy beam welding processes is that the high energy density of the focused laser or electron beam is able to produce deep narrow welds of minimal weld metal volume, enabling the formation of structural butt welds that add little additional weight and cause less component distortion in comparison to other welding techniques, such as arc welding processes. Additional advantages particularly associated with laser beam welding include the ability to be performed without a vacuum chamber or radiation shield usually required for electron beam welding. Consequently, laser beam welding can be a lower cost and more productive welding process as compared to electron beam welding.
Though filler materials have been used for certain applications and welding conditions, laser beam and electron beam welding processes are typically performed autogenously (no additional filler metal added). The high-energy beam is focused on the surface to be welded, for example, an interface (weld seam) between two components to be welded. During welding, the surface is sufficiently heated to vaporize a portion of the metal, creating a cavity (“keyhole”) that is subsequently filled by the molten material surrounding the cavity. A relatively recent breakthrough advancement in laser beam welding is the development of high-powered solid-state lasers, which as defined herein include power levels of greater than four kilowatts and especially ten kilowatts or more. Particular examples are solid-state lasers that use ytterbium oxide (Yb2O3) in disc form (Yb:YAG disc lasers) or as an internal coating in a fiber (Yb fiber lasers). These lasers are known to be capable of greatly increased efficiencies and power levels, for example, from approximately four kilowatts to over twenty kilowatts.
Hybrid laser arc welding (HLAW), also known as laser-hybrid welding, is a process that combines laser beam and arc welding techniques, such that both welding processes simultaneously occur in the same weld pool. The laser beam is typically oriented perpendicular to the surfaces to be welded, while the electric arc and filler metal of the arc welding process (for example, gas metal arc welding (GMAW, also known as metal inert gas (MIG) welding) or gas tungsten arc welding (GTAW, also known as tungsten inert gas (TIG) welding) are typically positioned behind and angled forward toward the focal point of the laser beam on the weld joint surfaces. This position of the arc welding process is known as a “forehand” technique. The benefit of the HLAW process is the ability to increase the depth of weld penetration and/or increase productivity by increasing the welding process travel speed, for example, by as much as four times faster than conventional arc welding processes.
Even though laser beam welding is known to have the various benefits noted above, deep penetrating laser beam welding techniques are known to be prone to trapped porosity. This propensity can be attributed to the low heat input associated with laser beam welding compared to typical fusion arc processes. As a result, the weld pool produced by laser beam welding tends to freeze very quickly, trapping gas-metal reaction products generated during the welding process. Though the inclusion of an arc process in HLAW processes helps to reduce porosity in shallow welds, for example, weld depths of less than one-half inch (about one centimeter), porosity resulting from trapped gas bubbles is an issue when attempting to achieve greater weld depths.
Reducing or eliminating porosity in deep laser welds would be particularly advantageous from the standpoint of achieving longer lives for components subjected to cyclic operations. One commercial example is the fabrication of wind turbine towers. Currently the use of welding processes that utilize a laser beam welding technique has been discouraged because of the propensity for large amounts of fine-sized internal porosity found in deep weldments produced by laser beam welding. The presence of porosity can significantly reduce the fatigue life of a weld joint and, therefore, a structure that contains the weld joint. Consequently, other welding techniques such as submerged arc welding (SAW) processes are more typically employed in the fabrication of structures subjected to cyclic operations, such as wind turbine towers. However, when used to weld large thick sections required in the construction of wind turbine towers, a significant drawback of the SAW process is low productivity, for example, resulting from the necessity to perform multiple passes at relatively low speeds, for example, about twenty to forty inches (about 50 to 100 cm) per minute. Though preheating the components just prior to welding might achieve a lower cooling rate to allow gas bubbles to escape the weld pool, in practice a component may require being heated to nearly three-quarters of its melting temperature, which is both expensive and can have deleterious effects on the base material properties of the component. Following laser beam welding with a second laser beam welding treatment to release the gas bubbles has also proven to be ineffective, since the weld pool produced by the second treatment also tends to freeze too quickly to allow gas bubbles to float free of the weld pool.